The presence of oxide (scale) on the surface of steel strip, sheet, or breakdowns is objectionable when such materials are to be further processed. For example, the oxide must be removed and a clean surface provided if satisfactory results are to be obtained from the hot-rolled sheet or strip in any operation involving deformation of the material. If the sheet is for drawing applications, removal of the oxide is essential, and its presence on the steel surface tends to shorten die life, cause irregular drawing conditions and destroy surface smoothness of the finished product. Oxide removal is also necessary to permit proper alloying or adherence of metallic coatings and satisfactory adherence when a non-metallic coating or paint is used. Moreover, in the production of cold-reduced steel sheet and strip, it is necessary that the oxide resulting from hot rolling the steel slab to breakdown form be completely removed before cold reduction to prevent lack of uniformity and eliminate surface irregularities.
The term "oxide" as used herein, unless otherwise stated, refers generally to the chemical compounds of iron and oxygen, as well as the chemical compounds of iron alloying elements, e.g., chromium, and oxygen, formed on the surface of the steel by exposure to air while the metal is at an elevated temperature. "Scale" is specifically the oxidized surface of steel produced during hot working of steel. Hence, the oxide produced on steel surfaces in hot-rolling processes is known as mill scale. Chemical compounds thus formed include iron oxides, such as FeO, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, chromium oxides, and the like.
Pickling is the process of chemically removing oxides and scale from the surface of a metal by the action of water solutions of inorganic acids. Considerable variation in type of pickling solution, operation and equipment is found in the industry. Among the types of pickling equipment may be mentioned the batch picklers, modified batch, non-or semi-continuous and continuous picklers.
The reaction occurring when steel or iron materials are immersed in dilute inorganic acid solutions includes the solution of metal as a salt of the acid and the evolution of hydrogen. Steel pickled in dilute hydrochloric acid and sulfuric-acid solutions is an example of this reaction, with the end products of reaction being, respectively, ferrous chloride and hydrogen and ferrous sulfate and hydrogen. Adherent films of oxides are undermined by the acid attack upon the scale on the base metal.
The rate of pickling is affected by numerous variables, including the steel-based constituents and type and adherence of oxide to be removed. Solution temperature and concentration, reaction product concentration, agitation, time of immersion and presence or absence of inhibitors and accelerators influence the rate of acid attack. Because of factors including pickling speed and efficiency, as well as reduced attack on the base metal, hydrochloric acid has effectively displaced sulfuric acid as the acid of choice in industrial pickling of carbon steel and other conventional, non-stainless, steels. While the rate of pickling increases in direct proportion to the concentration of the acid, the influence of temperature is much more pronounced. For example, in 15 per cent sulfuric acid an increase in temperature over the range 70.degree. F. to 210.degree. F. doubles the pickling rate for each rise of 15.degree. or 20.degree. F. in temperature. Rate of solution of iron at 180.degree. F. is about five times the rate at room temperature. Certain metals, such as copper, chromium and nickel, retard the rate of pickling when they occur in the steel base, since the scale bearing these alloying metals inhibits acid attack. Silicon and aluminum, for example, form refractory-type oxides, which in turn lower the solubility rate of the oxide in the acid.
With the advent of continuous cold-reduction mills, it was necessary to design and develop suitable equipment to remove the oxides resulting from the continuous hot-rolling operation and prepare the hot-rolled breakdowns for cold reduction in coil form. This operation is typically performed in either a continuous or semi-continuous (push-pull) pickling line. The primary function of a continuous and semi-continuous pickling line, as with other pickling processes, is the removal of oxide from the steel surface. This serves to promote maximum cold reduction with a minimum of power to assure good roll life in the cold-reduction mills within which the strip will be later processed and to secure the increased surface density possible with cold work. The primary differences between a continuous pickle line and a non-or semi-continuous pickle line is that in a continuous line the tail of one coil is welded to the head of the next coil so that the strip is always in tension. In addition, continuous pickle lines generally require looping pits for providing strip storage space when brief delays arise at the strip charging end and for permitting a uniform rate of travel through the pickling tanks. An advantage of semi-continuous picklers is that they readily accommodate coil-to-coil changes in strip width and gauge with minimal down time.
The thickness of the oxide varies considerably on steel rolled on the hot-strip mill. For example, loose coiling permits greater atmospheric penetration into the wraps, with corresponding heavier oxide formation on the edge areas. In addition, coiling temperature affects the adherence of the oxide and determines, in part, how easy or difficult it is to remove. Typically, hot coiling within a prescribed temperature window, e.g., 1150.degree.-1300.degree. F., makes oxide removal easier.
Conventional non-continuous and continuous pickling lines for carbon steel and other conventional non-stainless steels generally include the steps of uncoiling the steel strip coil, semi-flattening the strip in a processor comprising a series of five rolls, generally having diameters of 8-12 inches, to remove the coil set and to crack the surface scale at the strip edges, passing the strip through a secondary scalebreaker which may, for example, be a two-high temper mill to further scalebreak while, at the same time, reducing gauge thickness and elongating the strip, pre-cleaning the strip in a hot water or mild caustic spray, pickling the strip in a series of turbulent flow pickle tanks using dilute acid such as hydrochloric acid or sulfuric acid and an acid inhibitor to prevent base metal attack by the dilute acid after scale removal, rinsing the pickled strip with water, drying the pickled strip, inspecting the strip, electrostatically oiling the strip and recoiling the strip.
More specifically, at the coil entry end of a typical semi-continuous or a continuous pickling line for removing the oxide scale from hot rolled carbon steel strip, such as is disclosed in U.S. Pat. No. 5,354,383 -Bianchi, which discloses a semi-continuous pickling line, are facilities for handling and charging coiled product into the line. These usually consist of conveyors on which the coils are placed in proper sequence by overhead cranes, upenders in cases where the coil is delivered with the axis vertical, and a motor-driven integrated buggy and hoist for placing the coil in the uncoiling or pay-off equipment. The primary cold-working equipment, integral with the uncoiling equipment, consists of a mandrel on which the coil is placed, a hold-down roll, and a series of smaller diameter (about 8-12 inches) rolls. After the coil is charged on the mandrel and the lead-end entered into the small diameter rolls, the hold-down roll is brought down and pressure applied to the material. This action alternatively flexes the steel around the smaller diameter rolls, thus somewhat "breaking" the surface scale into numerous fine cracks, particularly at the strip edges, and increasing somewhat the available sub-oxide area for pickling acid attack. This flexing also cold works the steel enough to eliminate, in large part, the fluting (formation of creases when the steel is bent or otherwise deformed) tendencies of the hot-rolled steel. The group of small driven rolls immediately following the hold-down or breaker roll applies tension to the steel and also serves to sufficiently straighten it to remove the coil set. A stationary shear is located after the processor for the cropping and squaring of the coil ends.
In some pickling lines, an auxiliary or secondary scalebreaker is provided to break the scale even further than was achieved in the processor at the entry end, and thus increase the speed at which the line can be operated and still produce satisfactorily pickled strip. The secondary scalebreaker may be a roller-type machine, or it may be a two-high temper mill preceded and followed by a tension bridle at the entry and exit sides of the mill. Use of a two-high temper mill, for example, may result in extension of the strip on the order of 3 per cent and an increase in hardness of 3 to 5 points on the Rockwell B scale.
The pickling zone usually consists of several individual acid-proof tanks located in series, comprising an effective immersion length of about 150 to 300 feet. While many lines have from three to five tanks, each about 40 to 80 feet long, some lines have only one long tank, divided by weirs into four or five sections. The strip is completely submerged under several inches of liquid acid bath as it travels through the tank or series of tanks forming the pickling zone. Automatic acid controls are available which monitor the HCl content in one or more tank sections of the line and automatically add acid to maintain a preset concentration. In a typical line the acid is added in the third and fourth tank sections. The pickling solution then cascades over the top of the strip and flows counter to the direction of strip travel through shallow channels cut in the weirs between tank sections. The tanks are either of the shallow-bath type (about 15 inches in depth) or of the more traditional type (about four feet in depth and weir heights are decreased about an inch per weir from the exit to the entry ends of the tanks). In a typical four tank line, weir heights would be on the order of 40, 39 and 38 inches in height from the exit to the entry end of the pickling tanks.
In hot-rolling operations, the steel strip is usually water cooled after hot-rolling by exposing the top surface of the strip to a stream or spray of water. The cooling water tends to pool on the top surface of the strip, thereby cooling the top surface at a faster rate than the bottom surface. Since the rate of oxidation of steel is a proportional function of both time and temperature, scale formation is virtually always thicker on the bottom surface of the hot-rolled strip. To deal with this, conventional pickling tanks inject the acid from the bottom of the tanks and direct the acid upwardly toward the bottom side of the strip to enhance the acid attack on the relatively thicker scale at the strip bottom surface. The thinner and less developed oxide films at the strip top surface are pickled at the same rate as the bottom surface scale by the turbulence normally accompanying a strip travelling through a pickling bath.
Following the acid tanks in a conventional pickle line are rinsing tanks consisting of cold-water spray rinse and, occasionally, a hot-water tank. The cold water rinses the carry-over acid from the strip. The hot water rinse is a tank with an effective product immersion length of 15 to 20 feet. This tank completes the rinsing and by warming the steel, promotes flash drying prior to entering the succeeding set of pinch rolls. Situated between the final rinse tank and the pinch rolls are one, two or three banks of hot-air dryers operating at low pressures. Pinch rolls at the exit end of the pickling tanks control the speed of product travel and, in conjunction with the pinch rolls which provide back tension at the entry end of the line, help to maintain the proper loops in the tanks.
The delivery or exit end of the pickling line commonly has, in the order listed, a looping pit, pinch rolls, shear, oiler, recoiler and suitable supplementary equipment for conveying the finished product from the line. The pinch rolls preceding the shear are located so that product delivery to the shear is facilitated. Stitches, if present, are removed at this point, as well as short sections which inspection has shown to be of inferior quality. Some lines are provided also with rotary side trimmers at the entry end or, more commonly, at the delivery end.
Inspection of the raw pickled product, by simultaneous inspection of both surfaces of the strip using a battery of lights and one or more mirrors, is carried on continuously at the exit end of the pickling lines. Each coil is inspected for surface and edge quality, width and gauge. Some of the defects commonly causing rejection or diversion are slivers, cracked edges, laminations, off-gauge, off-width, roll marks, underpickling, overpickling, handling damage and pitting.
Underpickling results when the steel has not had sufficient time in the pickling tanks to become free of adherent scale and occurs when acid concentration, solution temperatures and line speed are not balanced properly. Variations in the oxide and composition of the steel are also factors in underpickled product, as well as such factors as coiling temperature of the hot-strip mill and inadequate amount of cold working through the processor. Overpickling results from the line delays which permit sections of the steel to remain in the acid too long. The presence of an inhibitor during conventional carbon steel pickling is essential to reduce iron loss. When an inhibitor is not used, iron loss during a short delay period appreciably reduces thickness of the carbon steel and raises the hazard of hydrogen embrittlement. Pitting is related to overpickling, the presence of nonmetallic inclusions near the steel surface and to rolled-in scale, slag or a refractory substance. While overpickling is not common in continuous or semi-continuous pickling operations, its occurrence does have a very serious effect on cold-reduction performance and surface appearance of the finished product. Furthermore, product damage from handling or improper equipment adjustment can render the steel unsuitable for further processing.
Prior to recoiling, the pickled steel passes between a set of oiling rolls which cover both surfaces with a small amount of oil. The type of oil used to lubricate the steel, and protect it from rusting during storage and from scratching during handling, is determined by the type of lubricating system on the cold-reduction mill unit. Hence, palm oil diluted with light mineral oil, is applied to the steel at the pickling line when a straight palm oil or a solution containing palm oil is used on the cold-reduction mill in which the strip is to be subsequently processed. Finally, the pickled and oiled product is recoiled on a conventional up-type or down-type coiler.
Like carbon steel, stainless steels also oxidize following hot rolling and coiling. However, because stainless steels are highly alloyed with such metals as chromium, manganese, silicon, and the like, the oxide layer formed on the surface of the hot rolled strip has a different chemical composition than the predominantly iron oxide film which forms on carbon steel strip. Moreover, it is well known that the oxide film formed on high chromium-containing stainless steel strip is very tightly adhering, which makes the descaling or pickling of such stainless steels very difficult as compared to carbon steels and the more conventional non-stainless steels. To achieve efficient and thorough surface oxide removal from such stainless steels, more severe processing techniques must be used which substantially increase processing time and operational costs. Frequently, to effect complete oxide scale removal, chemical pickling of stainless steel strip must be preceded by mechanical descaling, e.g., by shotblasting, which has the disadvantage that the strip surface is damaged by the shotblasting.
The pickling process most commonly used for stainless steel involves the use of a mixture of nitric acid and hydrofluoric acid, the mutual concentrations of which vary according to the type of plant, the type of steel to be pickled, its surface characteristics and its past processing history. Although the process enables excellent results to be obtained, it has the very serious drawback that it creates considerable and substantial ecological problems due to the use of these particular acids. In this respect hydrofluoric acid is extremely corrosive and a harmful environmental pollutant. Nitric acid is the source of highly polluting nitrogen oxide (NO.sub.x) vapors which are emitted into the atmosphere and which are highly aggressive towards metals and non-metals with which they come into contact. In addition, high nitrate levels exist in the wash water and in the spent baths and create a major disposal problem. The elimination of NO.sub.x vapors in the air and nitrates in the spent baths creates considerable plant operational problems, very high operational costs and no certainty that the solutions to these ecological problems will satisfy current government limitations and regulations. In the final analysis, the cost of building and operating nitric acid/hydrofluoric acid pickling plants for hot rolled stainless steel strip in an ecological safe and economic manner is highly problematic.
As a result there has been considerable interest in developing stainless steel pickling processes and plants which do not use either nitric acid or hydrofluoric acid and which are ecologically safe. U.S. Pat. No. 5,354,383 outlines a number of nitric acid-free processes which have been proposed as an alternative to the traditional stainless steel pickling process based on these acids. However, to date, no ecologically acceptable, commercially practical and economic alternative has yet been found and there is a continuing need for such a process.